Experimentally, electronic transport measurements through a graphene
sheet usually require contacts to metal electrodes, e.g., see an
illustration in Ref. [1]}. When tunneling from metal
reservoir to graphene occurs in a large area, the contact becomes
Ohmic and the area under the contact forms a substance which is a
hybrid between graphene and normal metal [2]}. Depending on
the nature of this substance, the system can be appropriately
considered as a graphene/graphene/graphene (GGG) structure or a
normal-conductor/graphene/normal-conductor (NGN) junction whose
contacts can be modeled by honeycomb or square lattices,
respectively. The ballistic transport through the NGN systems has
been investigated systematically in Refs.
[3]}, [2]}, [5]}.
